Silicon (Si) monocrystal has been used as material for power semiconductor elements that control high breakdown voltage and large current. Power semiconductor elements fall into several types and are used selectively according to the intended use. For example, since bipolar transistors and insulated gate bipolar transistors (IGBTs) cannot be switched at high speed although current density can be made larger, the use of bipolar transistors is limited up to several kHz and the use of IGBTs is limited up to about 20 kHz in frequency. On the other hand, power MOSFETs can be used at high speeds of several MHz although large current cannot be handled. Nonetheless, since a power device supporting both large current and high-speed performance is strongly demanded in the market, particular efforts are made to improve IGBTs and power MOSFETs, which have been developed substantially to the theoretical limits determined by the materials.
FIG. 12 is a cross-sectional view of a general MOSFET. An n− drift layer 102 is layered on an n+ substrate 101, and a p-base layer 103 is formed on the n− drift layer 102 with an n+ source layer 104 selectively formed in a surface layer of the p-base layer 103. A gate electrode 107 is formed on the n− drift layer 102 and the p-base layer 103 as well as the n+ source layer 104 via a gate insulation film 106.
Moreover, super-junction MOSFETs have recently attracted attention. FIGS. 13, 14, and 15 depict a cross-sectional structure of a typical element. For example, a super-junction MOSFET is disclosed in Non-Patent Literature 1 and is disclosed as a CoolMOSFET in Non-Patent Literature 2. With these techniques, a P-layer 110 is vertically formed into a columnar structure in an n− drift layer so as to dramatically improve ON-resistance without deterioration in breakdown voltage characteristics between source and drain.
Materials are also studied in terms of power semiconductor elements and, as disclosed in Non-Patent Literature 3, silicon carbide (SiC) is recently attracting attention as a next generation power semiconductor element and as a low ON-voltage element having excellent high-speed/high-temperature characteristics. Chemically, SiC is a very stable material with a wide band gap of 3 eV and can be used extremely stably as a semiconductor even at high temperatures. SiC also has a critical electric field that is 10-fold or greater than silicon. SiC is very likely to exceed the material limit of silicon and is therefore largely expected to grow in use as power semiconductors, for example, MOSFETs, in the future. Since SiC has small ON-resistance, it is expected to realize a vertical SiC-MOSFET having lower ON-resistance with high breakdown voltage characteristics maintained.
A cross-sectional structure of a typical SiC-MOSFET is the same as that of silicon (depicted in FIG. 12). The n+ source layer 104 is selectively formed in the surface layer of the p-base layer 103 laminated on the n− drift layer 102, and the gate electrode 107 is formed on the n− drift layer 102 and the p-base layer 103 as well as the n+ source layer 104 via the gate insulation film 106 with the drain electrode 108 formed on the back surface of the substrate 101.
An SiC-MOSFET formed in this way is an element switchable at high speed with low ON-resistance used as a switching device. For example, the SiC-MOSFET is expected to be utilized in power converters such as an inverter for motor control and an uninterruptible power supply (UPS). Since SiC is a wide band gap semiconductor material, SiC has critical electric field about ten times higher than silicon as described above and is expected to achieve sufficiently smaller ON-resistance.
A measure for achieving lower ON-resistance is also disclosed as a method of creating an element structure even in a region under a gate pad in an attempt to expand an effective element area (see, e.g., Patent Documents 1 to 5).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2010-177454    Patent Document 2: Japanese Laid-Open Patent Publication No. 2010-87126    Patent Document 3: Japanese Laid-Open Patent Publication No. 2009-105177    Patent Document 4: Japanese Laid-Open Patent Publication No. H8-102495    Patent Document 5: Japanese Laid-Open Patent Publication No. H4-239137    Non-patent Literature 1: Fujihira, et al, JJAP, Vol. 36, Part 1, No. 10, 1997, pp. 6254    Non-patent Literature 2: Deboy, et al, IEEE IEDM 1998, pp. 683    Non-patent Literature 3: IEEE Transaction on Electron Devices, Vol. 36, 1989, p. 1811